1. Field of the Invention
The present invention relates to a scanner lens, an image reader incorporating the scanner lens, and an image forming device incorporating the image reader.
2. Description of the Related Art
An image reader such as facsimile machine or digital copier is configured to reduce image information to read with a scanner lens and form an image thereof on a solid image sensor as CCD to convert the image information into signals. There is an optical system available which uses a 3-line CCD which is light receiving elements with red, green, blue filters arranged in three arrays on a single chip, to form an image of an original document in three original colors on the light receiving surface, thereby converting color image information into signals.
The scanner lens for use in such an optical system is required to exert a high contrast in a high spatial frequency range and about 100% aperture efficiency even in the periphery of angle of view. Further, to read color documents accurately, it is necessary to form red, green, and blue images at the same position along the optical axis on the light receiving surface and correct respective color aberrations precisely.
Moreover, there have been demands for the image reader not only to improve performance and function but also downsize and reduce manufacturing costs. For the purpose of downsizing the optical system, conjugation length needs to be shortened so that a lens with a wider angle of view is required while for cost reduction the number of lenses needs to be decreased.
For a scanner lens made up of a relatively small number of lenses and having a wide angle of view, Japanese Patent No. 3862446 (Reference 1) and Japanese Patent Application Publication No. 2002-244033 (Reference 2) disclose one with a half angle of view of about 30 degrees and made up of five groups, five lenses, and Japanese Patent No. 3939908 (Reference 3) and No. 4496231 (Reference 4) disclose one including an axisymmetric, aspheric lens, for example.
However, the lens disclosed in References 1 and 2 includes an anamorphic lens with different refractive power in directions orthogonal to each other, which is difficult to process, therefore expensive. Thus, cost reduction cannot be easily realized. Meanwhile, a half angle of view of the lens disclosed in References 3 and 4 is as narrow as 21.5 degrees so that the conjugation length of the optical system is large, which hinders downsizing.
Meanwhile, the scanner lens comprised of a small number of lenses cannot satisfy various demands for properly correcting axial color aberration in a wide range, field curvature at a very wide angle and various types of aberrations, for exerting very high aperture efficiency at high F-number, and for exerting high contrast in a high spatial frequency range.